1. Field of the Invention
This invention relates to a method of producing a dimensionally graded electroconductive composite material using a polyurethane foam that is coated with an electrically conductive polymer. The composite material that is formed can be used as an electromagnetically responsive bulk absorber. The conductivity gradient is achieved during a one time chemical reaction process that occurs over time.
2. Description of the Prior Art
Bulk absorber materials are useful for electromagnetic screening of aircraft structures and electronics from radar detection. They can also be used for the construction of radar range ground planes and serve as electromagnetically lossy filler material. Bulk absorbers can be made from an otherwise nonconductive material and preparing an electrically conductive composite by combining it with other, electrically conducting components.
Traditionally, nonconductive materials, for example polymeric materials, have been made electrically conductive by blending or loading the nonconductive material with such conductors as electroactive graphite, carbon black, metal oxides, metal particles and others. Often, such electroactive components consist of metal powder, flakes, wires, thin pattern metal depositions or metal sputtering. In order to obtain the desired electrical activity in the composite material, high particle content may be required, typically as much as 40 percent by weight. When a high conducting particle content is present, the mechanical and physical properties of the resulting composite material may become severely affected. When metallic particles or loadings are involved, a deterioration of the mechanical stability and delamination of the composite structure may occur due to the poor polymer to metal adhesion in the loaded composite/laminate. The high particle content usually increases the weight of the electrically active composite materials significantly. When the composite material is to be used in applications where added weight is undesirable, such as in aircraft, this can be a serious disadvantage.
The preparation process of a highly loaded polymer/metal or polymer/graphite composite is difficult and time consuming, and requires the use of specialized processing equipment. In effect, these polymer/metal or polymer/carbon black composite absorbers, with or without a conductivity gradient, are mechanically weak, heavy by weight, difficult to prepare and expensive.
Presently, a compatible to this invention, commercially available bulk absorber based on polyurethane foam can be produced by coating the open cell polyurethane foam with a carbon black powder. This bulk absorber is used on a large scale in anechoic chambers, radar/radio frequency testing facilities, and electromagnetic (EM) screening of buildings and devices. The bulk absorber is produced by soaking polyurethane foam in a carbon black slurry (ink) and then drying the foam. The amount of deposited carbon is measured by weight. The resulting electrical activity and the electromagnetic transmission loss through the coated foam depends on the amount/weight of deposited carbon black. The correlation between the amount of deposited carbon black and the resulting transmission loss is experimentally established.
The carbon black ink slurry contains other chemical ingredients to permit the adhesion of carbon black powder on the foam surfaces. Usually, polyurethane foam of one inch thickness is handled this way, although thicker foams can be coated. When higher transmission loss is required, thicker coatings of carbon black are applied by repeating the coating and drying process on the foam. Significant amounts of carbon black are then deposited on the foams. With higher carbon black loadings, the resulting foam composites are heavy by weight and stiff. The carbon black coating easily crumbles off the foam. Coated foams with 0-30 dB of electromagnetic loss can be produced.
A dimensional conductivity gradient in the polyurethane/carbon black foam can be obtained by combining (gluing) fragments of coated foams prepared previously, each of varying conductivity/electromagnetic transmission loss value. It is not possible to grade the amount of the electroactive coating on one continuous piece of polyurethane foam, using the carbon black slurry ink method. A new, improved method of producing a light weight polyurethane bulk absorber is described in this invention.
Some of the problems of preparing composite bulk absorber materials have been circumvented by developing composite materials using electroconductive polymers. Electroconductive polymers, such as polypyrrole, have been known for about 20 years and can be prepared in electrochemical and chemical reactions as films or powders. The electrical conductivity achieved in these polymers can be more than 1,000 ohm.sup.-1 cm.sup.-1. The chemically and physically stable conducting polypyrrole can be readily prepared. While the electroconductivity of polymers is an attractive attribute, their non-processability due to general lack of solubility in common solvents presents a major drawback. Free standing conductive polymer films are often brittle, thus their industrial utility is limited. These limitations can be circumvented by preparing flexible and stable electroconducting polymer/polymer composites that can be prepared in a variety of ways.
Electroconductive composites may be prepared by polymerizing an electroconductive polymer onto the existing structure of a nonconductive material. Such composites have been prepared using a variety of compounds and methods. Both electrodeposition and chemical oxidation methods can be used to produce these electroconducting composite materials.
Conducting composite films that have been prepared electrochemically include polypyrrole/polyvinyl chloride, polypyrrole/polyurethane and polyaniline/polyurethane among others. These composite materials were prepared in the form of thin films on the surface of an anode. The nonconductive polymer host was first dissolved and coated on the surface of the anode prior to electrodeposition of the conductive polymer into the host's matrix. Other soluble polymers can be impregnated with a conducting polymer network in a similar fashion. A flexible and stable composite film can be obtained this way. The evidence suggests that the resulting composites combine the advantageous properties of the polymer host (mechanical stability, flexibility and overall durability) and the conductivity of the interpenetrating conducting polymer network.
Of the electroconductive polymers known to date, polypyrrole has been found to be one of the best candidates for the preparation of electroconductive composites with other materials. Polypyrrole is an extremely stable material with good chemical, mechanical and electrical stability that changes little over time.
The chemical polymerization process can also be used to deposit conductive polymers on the surfaces and inside the structures of nonconductive materials. This generally involves contacting the nonconductive host with such polymerizable compounds as pyrrole or aniline or their derivatives. These monomer compounds can be polymerized to form an electroconductive polymer that forms as a deposit on the nonconductive substrate. The following patents disclose the preparation of electroconducting composites by a chemical deposition of polypyrrole on nonconducting substrates.
U.S. Pat. No. 4,604,427 describes a chemical method of preparing an electronically conducting polymer blend by impregnating a non-porous, swellable or soluble polymer host with pyrrole or aniline or their derivative monomers, and oxidizing the monomers to form electroconducting polymers within the host polymer network. Using this method, polymeric materials can be impregnated by a conducting polymer up to one millimeter deep and impart electrical conductivity to the surface layer of nonconducting non-porous polymers. Other porous materials can be made conducting by depositing electroconducting polymers on their surfaces.
U.S. Pat. No. 4,617,228 describes and gives examples of the preparation of the electroconducting composites comprised of fiberglass fabric coated with the electroconducting polypyrrole, in organic solvents. This patent claims that all other dielectric porous materials can be used to prepare electroconducting composites with polypyrrole in a similar process or repeated process treatment if higher conductivity levels are desired.
U.S. Pat. Nos. 4,803,096 and 4,877,646 describe the preparation of electrically conducting fabrics by depositing coherent polypyrrole coating on surfaces of fabric fibers. This process took place in dilute water solutions, with the use of complexing agents to aid the uniformity of the coatings. Fiberglass, natural and synthetic organic fibers could be coated by this method. Numerous examples presented in these two patents show how various reaction parameters (concentrations of reactants, reaction time, use of surface active agents, reaction temperature, choice of oxidant, doping agent, etc. . . . ) affect the uniformity and conductivity levels of the polypyrrole coating. The conductivity of fabrics was measured by two parallel, two inch long electrodes spaced one inch apart on the surface of the fabric. This measurement was expressed as surface resistivity (ohms per square). A range of 50 to 500,000 ohms per square was measured on surfaces of fabrics described in these patents.
While the above examples describe methods of forming a uniform electroconductive polymeric coating on dielectric substrates, they do not show a dimensionally graded electroconductive polymeric deposit upon the substrate or host.